The invention concerns a backpressure grid for pressurized filtration systems.
The term "pressurized filtration" as used in connection with the invention includes filtration-like processes, for example, and especially, a reversed osmosis process.
Pressurized filtration systems of the type under discussion comprise usually pressure vessels in two or more parts which contain in an upper housing section at least one inlet for the fluid to be treated, and in a lower housing section at least one outlet for the permeation product. The two housing sections, connected pressure-proof with each other, are divided from each other by a usually horizontally arranged filter-supporting plate, with the filter, usually a membrane, resting upon it. As a rule, this filter consists of several layers, one upon the other.
In case of the standard pressurized filtration systems a grid or sieve, called a backpressure grid, is arranged closely above the surface of the filter. This backpressure grid has the task to prevent in the case of a reversal of the pressures acting upon the membrane, for example, from a damming-up and/or a pressure drop at the inflow side, a lifting, or at least an excessive lifting of the filter from its support which would lead to damage to the filter.
In the case of standard pressurized filtration systems this backpressure grid is designed in the form of a metal mesh screen, normally a stretched metal sieve, usually coated with a layer of PTFE (polytetrafluoroethylene). This layer will prevent the filter from adhering to the screen, thus protecting the filter or membrane respectively from damage, especially during autoclave operations. The coating serves also as corrosion inhibitor.
In order to prevent the inflowing stream, which is rather powerful in the case of pressurized filtration systems under discussion, from striking the filter directly and damaging the same, there is usually placed between the entrance of the inflow line into the pressure chamber of the pressurized filtration system and the backpressure grid a diffusing disk, called a baffle plate. The diameter of this, usually circular, baffle plate is designed as small as possible and at the same time as large as necessary to break, and distribute radially, the axially fed inflow thus protecting the membrane or the filter respectively.
In the case of the known pressurized filtration systems of this type the baffle plate is bolted or welded by way of web fittings or holding screws either to the upper housing section of the pressurized filtration system or directly to the backpressure grid.
The known pressurized filtration systems are disadvantageous because they do require the use of a baffle plate. Such baffle plate represents an additional component which must be installed by expenditure of additional labor. It is a structural member which is bulky due to its installation which requires establishment of minimum distances and is another component requiring maintenance and cleaning. Finally, the installation of the baffle plate in the pressure chamber of the pressure vessel requires the use of fastening points, that is weld joints, screw connections or riveted joints which are open to attack by corrosion.
Furthermore, tests conducted by the use of colored or otherwise marked fluids to be treated have shown that, surprisingly, baffle plates of all standard sizes will not distribute the inflow uniformly over the unobstructed surface of the filter. Filters of all diameters will display in all instances the "shadow" of the baffle plate, usually at the center of the circular filter, i.e. an area below the baffle plate which is practically not loaded at all by the fluid treated, especially in case of filtrations. This area on the filter is enclosed concentrically by an area which extends radially outward, its load far above average relative to the total active surface of the filter. Surprisingly, this overloaded radial central area is followed, especially in the case of filters with larger diameters, by a relatively wide peripheral area of the filter surface which is not utilized at all.
In view of this present state of the art it is the object of the invention to provide a backpressure grid which is designed in such manner that it will allow the construction and operation of a pressurized filtration system without the use of a baffle plate and will insure at the same time, even in the case of filters with large diameters, a homogenous distribution of the inflow over the entire working surface of the filter.
In order to solve this problem, the invention proposes that the backpressure grid be designed in the form of a perforated plate, all openings being very small and equal in size and distributed unevenly over the area of the perforated plate in such manner that the surface distribution density of the openings (number of openings per unit area) is substantially lower within the impact area of the inflow than within the immediately adjacent area of the perforated plate surrounding the impact area as a proximity area, and that the surface distribution density of the openings within the outer area, surrounding the proximity area, is again lower but still higher than within the impact area.
The backpressure grid proposed by the invention makes the use of a baffle plate unnecessary, even in the case of a very thinly constructed pressurized filtration system where the inflow aperture is located immediately above the surface of the backpressure grid. Since the backpressure grid has only a few openings of relatively small diameter within the impact area of the inflow the filter section located below the inflow aperture will be fully protected even at continuous operations. On the other hand, the arrangment of at least a few openings in the backpressure grid even within the area opposite to the inflow makes certain that even the areas of the working surface of the filter directly opposed to this area will become fully operative, not being located within the "shadow" of a baffle plate any longer.
The novel backpressure grid offers the additional substantial advantage that is possesses openings of very small diameter distributed over its entire area and that the entire unobstructed pass-through profile, made available by the openings, is very small relative to the total area of the backpressure grid plate, preferably in the range of 1 to 2% of the working surface of the backpressure grid. This will cause a certain damming-up at the backpressure grid by the fluid to be treated, resulting in a homogenous radial distribution of the feed fluid to be treated at the backpressure grid and thereby throughout the surface of the filter. The use of openings with identical diameters for the entire backpressure grid simplifies the manufacture of such unit. The specific uneven distribution of these openings over the working surface of the backpressure grid will compensate the rates of flow existing within the pressurized filtration system in such manner that the medium to be treated will arrive at the available filter surface in practically homogenous distribution. The higher surface distribution density of the openings within the proximity area which surrounds, and borders directly at the impact area will prevent the feed from radially skipping said area as a result of the deflection of the inflow striking the backpressure grid. Otherwise only an insufficient proportion of the medium to be treated would reach the surfaces of the filter which are located directly below this proximity area. The backpressure grid plate of the present invention reduces the surfaces distribution density in the peripheral outer areas where the back pressure of the feed flow, building up at the outer housing walls, has a greater influence.
Due to this compensation of the flow conditions within the pressure chamber of the pressurized filtration system, the backpressure grid proposed by the invention makes it thus feasible to omit the baffle plate which is indispensable in the case of known standard systems while insuring in addition thereto a practically complete homogenous distribution of the fluid to be treated throughout the working surface of the filter. In this manner there is guaranteed a full utilization and loading of the available filter surface area under any circumstances and especially for filters possessing large working diameters. This is particularly essential and important if membranes are used where a transverse flow component of the filtrate is practically not operative.
The backpressure grid of the invention is preferably made of high-grade steel and coated with an anti-adhesive protective layer, preferably consisting of polytetrafluoro ethylene. The openings are preferably produced by photo-etching, boring with a Laser beam or an electron beam in order to avoid any flash formation. These techniques are known per se and familiar to any person skilled in the art. Etching by use of photoresist masking was found to be an excellent method to produce the regular diameters of 0.5 to 1.5 mm and preferably 1.0 mm for the openings of the backpressure grid. In any case, however, the diameter of the openings is substantially greater than the diameter of the largest particles to be seperated from the feed liquid. Coating of the plate is effected in a step following the cutting of said bores or openings so that also the inner walls of said openings are entirely covered by said protective coating.